Method for deriving a route through a network

ABSTRACT

A method of deriving a route through a network comprises analysing constraints imposed by a stream of data to be routed through the network; calculating a best path through the network between a first node (A) and a second node (G); and setting up, between the first and second nodes, a bearer trail which satisfies the constraints. The method further comprises analysing the chosen route using a state machine to ensure compliance before defining the bearer trail.

[0001] This invention relates to a method of deriving a route through anetwork, in particular for optical networks.

[0002] In an optical network, nodes at an input and an output may not bedirectly connected. For the purpose of this application, a connectionbetween two nodes in a network which are not directly connected to eachother is known as a bearer trail. The bearer trail starts and finishesat multiplexing devices.

[0003] A conventional method of routing through a network is known asmulti-protocol label switching. The method operates by creating a labelswitched tunnel through the network, this tunnel or bearer trail iscreated between the two nodes described above. However, a disadvantageof this system is that a bearer trail is set up in advance of any datatransmission and there may not be any traffic which requires theparticular connection to be created. This results in additional costs,reduced efficiency and limits the number of users, due to the redundancyin the equipment.

[0004] In accordance with the present invention a method of deriving aroute through a network comprises analysing constraints imposed by astream of data to be routed through the network; calculating a best paththrough the network between a first node and a second node; and settingup, between the first and second nodes, a bearer trail which satisfiesthe constraints; wherein the method further comprises analysing thechosen route using a state machine to ensure compliance before definingthe bearer trail.

[0005] The present invention is more efficient and cost effective thanprior art methods. A bearer trail is only set up when required inresponse to a data stream which will use that link, so there is nounnecessary redundancy. A further feature of the present invention isthat the state machine enables the route to be tested for allconstraints before it is defined. Once defined the link is fixed, sothis is another feature which avoids undue redundancy.

[0006] Preferably, the method further comprises analysing constraintsfor subsequent data streams and routing the subsequent data streams viathe existing bearer trail if the bearer trail satisfies the newconstraints.

[0007] The link that has been set up in response to one data stream isthen preferentially allocated to subsequent data streams for which itsatisfies the constraints. This reduces the overhead in setting up newbearer trails.

[0008] The invention is applicable to various networks, such as anynetwork containing areas of differing signal type capability orbandwidths, but preferably, the network comprises an automaticallyswitched optical network.

[0009] An example of a method of routing a path in a network accordingto the present invention will now be described with reference to theaccompanying drawing in which:

[0010]FIG. 1 illustrates possible connections in a network in which thepaths are routed by a method according to the invention;

[0011]FIG. 2 illustrates a path allocated according to the method of thepresent invention; and,

[0012]FIG. 3 is a state machine illustrating the connections of FIG. 2.

[0013] An example network is shown in FIG. 1. The network comprises 7nodes, A to G. Nodes A and G are ingress and egress devices forconnecting to a client via a 2.5 Gb connection and nodes B, C and Finclude multiplexing devices 1, 2, 3 and optical cross-connect (OXC) 4,5, 6. Both sets of nodes are outside a 10 Gb boundary. Nodes D and E arewithin the 10 Gb boundary and connect to their respective multiplexersvia a 10 Gb connection.

[0014] Before any channels inside an optical multiplexer (MUX) are usedthere is some flexibility concerning the node in the network that willcontain a corresponding de-multiplexer. In this example, node B'smultiplexer 1 can connect to either node C's multiplexer 2 or node F'smultiplexer 3. Node C's multiplexer can connect to either node B's ornode F's. This is possible because the cross-connect in node D canswitch the 10 Gb MUX connection from node B to either node C or node E(and hence node F). The 10 Gb boundary indicates that everything insidethis boundary is switched at the 10 Gb level. There is node-multiplexing capability to the 2.5 Gb level.

[0015] The different MUX connection possibilities are taken into accountby a routing algorithm such as constrained shortest path first (CSPF),open shortest path first (OSPF), intermediate system—intermediate system(IS-IS) during a route calculation phase in which the ingress node, nodeA, is calculating the best path through the network. The presentinvention is suitable for use with some standard routing algorithms suchas OSPF/IS-IS, but CSPF is preferred. Once a MUX connection (bearertrail) has been set-up between two nodes, this flexibility is lost. Inthis example, if the signalling protocols set up a MUX connection fromnode B to node F, then there is no longer a possibility of connecting Bto C using the MUX.

[0016] When a bearer trail is set up, a logical link is created torepresent this bearer trail through the network. This link is depictedin FIG. 2 and is labelled logical link 7. From then on, when there is arequest for a 2.5 Gb connection from A to G, the routing algorithm willsee the logical link 7 from node B straight to node F. This link willbecome the shortest 2.5 Gb path from B to F.

[0017] This logical link has the same type of properties as physicallinks in the network, such as signal type support, bandwidth constraintsand shared risk groups. Shared risk groups (SRG) are identifiers forphysical risks such as fibres, cables and ducts. A physical link in anetwork uses a set of these risks. In the case of the logical link, theSRGs for the link are the union of all the SRGs used by the individualhops taken by the logical links.

[0018] During the signalling of future paths using the bearer trail, theintermediate nodes (D and E in this example) do not need to bereconfigured. The usage of channels inside the bearer trail is invisibleto these nodes. The channel used for a path needs to be co-ordinatedbetween the multiplexing peers. This can be done at the signallingphase. It is not required that all the nodes in the network domain knowexactly which channels are used inside a bearer trail.

[0019] This provides the concept of pairing multiplexers during path setup to initialise a bearer trail through the network. This model can beextended to support any other type of multiplexing/bearer trailtechnology.

[0020] A state machine is used to model the process of going through amultiplexing device in the network. A simple example of the statemachine is shown in FIG. 3. In this example a network containing a 40 Gbboundary can also be supported. When a constrained routing algorithm iscalculating the route through the network, there must be a state changewhen using a multiplexing device. Before reaching the egress port at theegress node, the signal must be de-multiplexed to the original bandwidth(2.5 Gb in this case).

[0021] This state machine can be extended to support changing of signaltypes through the network as well as multiplexing devices. For example,once a synchronous optical network (SONET) device has been used allother devices must be SONET until a transport signal converter isreached. At this point it is possible to leave one signal boundary(SONET boundary) and enter another (e.g. synchronous datahierarchy—SDH).

[0022] The state machine is used during the route calculation phase ofpath set up. The process of calculating a route through a networkinvolves a manager requesting that a route is set up through a network,and supplying constraints relating to the path to the ingress node suchas signal type and bandwidth requirements. The ingress node thencalculates a route through the network using a routing algorithm whichsatisfies the constraints supplied by management. It is at this phasethat the state machine is used to make sure that the constraints (e.g.signal type or bandwidth) are fulfilled by each link in the route beingcalculated. The calculated path is then signalled through the network,and configured by each intermediate hop.

[0023] The information distributed by a route distribution protocol(e.g. OSPF/IS-IS) needs to include the multiplexing and de-multiplexingcapability of a link. In a link state protocol, such as OSPF, links areuni-directional. So for a bi-directional link, two links are actuallyadvertised. One link from A-B, another link from B-A. In some casesmultiplexing and de-multiplexing capabilities advertised in onedirection would be mirrored in the link advertised in the oppositedirection. This does not have to be the case. It is possible for thecapabilities to be different in either direction.

[0024] This model is extensible and not constrained to the multiplexingdevices used in the example. Any multiplexing equipment can be supportedusing the signal boundary concept. The state machine can also supportany signal constraint, such as bandwidth as described in the example,signal type, or any other constraint that can be applied to a path.

[0025] The present invention is applicable to any type of network whichcontains boundaries, such as signal type support or bandwidth, but isparticularly suitable for increasing efficiency of utilisation ofautomatically switched optical networks.

1. A method of deriving a route through a network, the method comprisinganalysing constraints imposed by a stream of data to be routed throughthe network; calculating a best path through the network between a firstnode and a second node; and setting up, between the first and secondnodes, a bearer trail which satisfies the constraints, the methodfurther comprising analysing the chosen route using a state machine toensure compliance before defining the bearer trail.
 2. A methodaccording to claim 1, further comprising analysing constraints forsubsequent data streams and routing the subsequent data streams via theexisting bearer trail if the bearer trail satisfies the new constraints.3. A method according to claim 1 or claim 2, wherein the networkcomprises an automatically switched optical network.